My family has a cabin in CO at 9,500 feet. There is a great spring up the road where we get our water, and the cabin has a basement that is in the low 50's in the summer and never gets below 34 in the winter.
I had an idea to brew a lager in July, bottle it late August and we would have brew for Elk hunting in the fall.
Aside from finding what I need to do with the water, is there any thing I should consider in relation to the elevation? A certain yeast or malt that can handle the lower 02 density in the air?

Never had a problem brewing in Breckenridge, CO... Boil is lower temp, chilling is easier with the colder ground water. Fermentation seems the same. Carbonation is a little off, and I had to add more suger to carb to good levels. Used a bunch of different yeasts. Let me know what specific questions you have...

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As mentioned above, the boiling temperature will be lower. Hop isomerization rate is directly to temperature. Therefore, the bittering equations are likely to underpredict the bittering contributions. You will have to either hop more or boil more to make up for that deficiency.